Numerical investigation of effects of rotating downdraft on tornado-like-vortex characteristics

Appropriate modeling of a tornado-like vortex is a prerequisite when studying the near-ground wind characteristics of a tornado and tornado-indrieed wind loads on structures. Both Ward- and ISU-type tornado simulators employ guide vanes to induce angular momentum to converge flow in order to generate tornado-like vortices. But in the Ward-type simulator, the guide vanes are mounted near the ground while in the ISU-type they are located at a high position to allow vertical circulation of flow that creates a rotating downdraft to generate a tornado-like vortex. In this study, numerical simulations were performed to reproduce tornado-like vortices using both Ward-type and ISU-type tornado simulators, from which the effects of rotating downdraft on the vortex characteristics were clarified. Particular attention was devoted to the wander of tornado-like vortices, and their dependences on swirl ratio and fetch length were investigated. The present study showed that the dynamic vortex structure depends significantly on the vortex-generating mechanism, although the time averaged structure remains similar. This feature should be taken into consideration when tornado-like-vortex simulators are utilized to investigate tornado-induced wind forces on structures.